TB Testing

Guidelines for Tuberculosis Screening

Whom to Test

TB Screening Performed by the Health Department

• Contacts to tuberculosis cases and suspects
• Special epidemiological circumstances
• Diagnostic purposes (symptoms suggestive of TB, special risk factors)
• Persons with AIDS; reactive HIV antibody test
• Health Department employees (two-step skin test upon employment)

In addition to those listed above who will be tested by health departments, the following persons and groups are recommended to have skin testing by private physicians and institutions. If these institutions and groups have skin testing programs, the health department will provide technical consultation.

• In Institutions where the employees are at relatively high risk of acquiring or transmitting disease the employees should be tuberculin tested periodically. This includes hospitals, prison, day-care centers, nursing homes, mental institutions, and such facilities.
• The routine periodic skin testing of school teachers, beauticians, and food handlers is specifically discouraged; however if local laws or regulations require these tests, they may be done by the patient’s private medical doctor.
  

How to Give the Tuberculin Skin Test

• The Mantoux PPD test will be administered as 5 tuberculin units, Tween 80 stabilized (0.1 ml). Skin testing may be done simultaneously with live virus vaccination.  If live virus vaccine is administered other than simultaneously with the tuberculin skin test, then a waiting period of 4 to 6 weeks should be observed before performing the test.

Interpretation of the Skin Test

• Skin tests will be read at 48 to 72 hours following administration. Results will be recorded in millimeters of induration measuring the transverse diameter of the reaction.
• 15 mm of induration or greater is considered significant for all persons in the general population, including children with no known contact or other risk factors.
• 10 mm of induration or greater is considered significant in: Foreign-born persons; Substance abusers (alcohol & intravenous drug abusers who are HIV negative); Residents of correctional institutions and nursing homes; Persons over age 70; Low income populations (including homeless); Nursing home and hospital employees, health department employees, and persons with certain medical conditions (Diabetes, Post gastrectomy, silicosis, corticosteroid therapy, etc.).
• 5 mm of induration or greater is considered significant in: Contacts; Immunosuppressed persons (HIV infected; AIDS); persons in immunosuppressive therapy; Persons with Hodgkins or leukemia, cancer, end-stage renal disease, persons with chest film abnormalities compatible with TB and persons who inject drugs (if HIV status is unknown).

What to Do with a Significant Skin Test (Measuring the transverse diameter of the reaction)

• All individuals with a significant skin test will be followed according to established guidelines depending on their category.
• In special situations such as screening nursing home residents and employees, and hospital employees, the two-step skin testing procedure should be utilized upon employment.  These two step procedures are included in the sections on Recommendations for Nursing Homes and Hospitals.

Screening Recommendations and Risk Questionnaire for Students

• Approved by TB Medical Advisory Council, January 30, 2009
• Approved by State Committee of Public Health, March 18, 2009

TB Blood Test

The TB blood test (interferon gamma release assay [IGRA]) mixes a patient’s blood samples with peptides that simulate antigens derived from TB bacteria and with controls. The TB antigens were chosen because they are found in M. tuberculosis complex, including M. bovis, but they are absent from BCG and from the majority of other mycobacteria.

In most people infected with TB bacteria, specific immunity develops within six to eight weeks. Then, some white blood cells can respond to the simulated TB antigens releasing interferon-gamma (IFN-γ). The tests measure the level of IFN-γ response. Control substances are used for comparison purposes to help verify test results and to determine a person’s background level of IFN-γ.

The U.S. Food and Drug Administration (FDA) has approved these two TB blood tests that are commercially available in the United States:

• QuantiFERON®-TB Gold Plus (QFT-Plus)
• T-SPOT®.TB test (T-Spot)

Interpretation of TB Blood Test Results

A positive test result usually means TB infection. More tests, such as a chest radiograph, are needed to rule out TB disease.

A negative test result means TB infection is unlikely, but cannot be excluded, especially if the patient:

• Has signs and symptoms consistent with TB disease or
• Has a high risk for developing TB disease once infected with TB bacteria (e.g., the patient is immunocompromised)?

A borderline, indeterminate (QFT-Plus only), or invalid (T-spot only) test result means the test did not provide useful information about the likelihood of TB infection. Repeating a TB blood test or performing a TB skin test may be useful.

Visit the CDC for more information regarding the TB Blood test.